Japetus is in synchronised rotation with Saturn. From Saturn, two hemispheres are therefore distinguishable, one hidden and one visible.
But we can also define two other hemispheres:
- The front hemisphere: Is the one that is always directed towards the satellites' direction of.
- The back hemisphere : Is the one that is always directed in the opposite direction.
The polar caps therefore have one half located in the front hemisphere and the other half is in the back hemisphere.
The front hemisphere of Japetus, except the polar regions, is dark with a very small albedo of about 0.05
and has a brown-reddish colour, the back hemisphere and the polar regions is very shiny with an albedo of 0.5 and a white colour.
The dark materials are carbonated materials and the shiny materials iced ones.
The origin of the dichotomy is not yet known. Two hypotheses exist :
- The dark materials would be particles ripped off one of Saturn's satellite, Phoebe, by meteorites. These particles would have then landed on Japetus' surface.
But actually, the colour of the particles aren't the same colour as Phoebe's surface.
- The dark materials could be from old cryovolcanic eruptions that would have brought to the surface components
of different a nature from the ones on the surface. These eruptions could also have a link to the creation of the equatorial mountain range.
The craters :
Japetus' surface is heavily cratered which means it is very old. Its age is estimated of 4.5 billion years.
A big impact basin is visible (image 3)
in the dark hemisphere,
and it is covered of small more recent craters (image 4)
The seasonal cycle :
The Cassini probe has shown a presence of carbon dioxide on Japetus' surface. The satellite has an inclination from the Sun of more or less
15°, which induces a seasonal cycle during which the CO2
to go from one pole to the other. The ice caps of CO2
change this way must not be confused with the shiny icecaps of water. The ice water caps are simultaneously represent on
the two poles while only one of the icecaps of CO2
exists today. The thickness of the CO2
caps doesn't rise above a few
m). During this trip back and forth from one pole to another, a small part (5 %) of the CO2
would be lost in space, therefore in less than 2 centuries, more than half of the CO2
of the surface would be lost.
The equatorial mountain range:
The equatorial mountain range sometimes rises to an altitude of 20 km and measures more than 1300 km long.
The way the impact craters are located on the range is similar to the way they are located on the rest of the surface.
The equatorial range must therefore be as old as the surface.
To explain its formation, several scenarios have been imagined but none have been confirmed.
- The scenario of the "synchronised rotation": The equatorial range would have been the result of tectonics created by the deceleration
of the rotation of the satellite during its synchronisation. If we suppose that the inside of Jepetus was hot and partially melted
while it is still in rapid rotation, its shape should be flattened because of the centrifugal force. During the deceleration
the satellite has to take a more spherical shape because of the reduction in centrifugal force. But meanwhile the thick lithosphere
would have cooled down and stiffened, which would oppose itself to this change of shape, thereby creating a fracture along the equator.
- The "impacts" scenario: just after the creation of Japetus, a disk of material stayed around the satellite.
The accretion would would have continued, creating an equatorial mountain range.
- The "convective" scenario: The range would be the result of the forces acting
above an ascending currant that is part of a movement of "solid" convection in a only slight malleable mantel (which means
in a similar way as the movement of the lithospheric plates that move because of the movement of the mental).
In the first and second cases, different internal structures are needed, just like a hot inside after the formation of the satellite.
This heat could have been given by the radioactive disintegration in a metallic core. A warm interior
could also be at the origin of cryovolcanism responsible of the dichotomy of the surface.
Figures 1 to 4(credit: JPL/NASA)